The use of screw threads to connect joints of pipe together so as to convey fluid, is a very old art that has progressed for hundreds of years in an effort to satisfy periodic needs for stronger and better sealing pipe connections. Performance requirements for pipe connections still vary widely today, such as for home piping with less than 80 psi fluid pressure and virtually no mechanical loads, to Oil Well Pipe that may be required to hold over 15,000 psi gas pressure and simultaneously, withstand extreme mechanical loadings and wide temperature fluctuations.
Due to the historical weakness of threaded pipe connections and their tendency to loosen, leak, and or break, their use in industrial plants and refineries has been limited by Industrial Codes to very small pipe sizes and low pressures. However, because there is no reasonable alternative pipe connection for use within the very limited hole sizes drilled for Oil and Gas Wells, threaded pipe connections are still used today, so most research and development of pipe connections has been directed toward such use.
In 1939 API adopted the 8Rd thread connection to connect joints of API tubing and casing which is still used to connect about 80% of well pipe today. My U.S. Pat. No. 5,427,418 filed in 1992 explained why API 8Rd Connections loosen and leak, and xe2x80x9cAPI Item 2239 Work Groupxe2x80x9d discovered that fact in 1995 and recently adopted some principles of ""418 in API xe2x80x9cSR17 Supplemental Requirements for API LTC Connections with Specified Performancexe2x80x9d.
In an effort to provide pipe connections that sealed better than API 8Rd Connections, special xe2x80x9cPremiumxe2x80x9d pipe connections were developed by numerous other parties who adopted thin annular sealing lips at the end of their pin threads as depicted in my U.S. Pat. No. 2,766,999 which was itself, a variation of my U.S. Pat. No. 2,766,829 which introduced proven reliable pressure-aided, high-pressure, high temperature metal seals to the nuclear and space industries as well as the oil and gas industry, which they now describe as xe2x80x9cmetal-to-metal sealsxe2x80x9d. However, such seals when used in oil-well pipe connections suffer a loss of connection efficiency because the pipe wall thickness for supporting axial loads is reduced to form the lip and it""s mating sealing surface, when formed in non-upset pipe ends.
There has been considerable confusion in the industry as to what constitutes a reliable qualification test for threaded pipe connections, which has resulted in too many sales claims reflecting hopes more than facts. New standard ISO-13679 gives promise to end that problem in that it allows one to choose the % efficiency ratings relative to the pipe ratings that a connection is to be tested and qualified for, under combinations of: internal pressure; external pressure; tension; compression; bending; temperature; and the choice of water or gas as the pressurizing fluid. It also specifies test procedures intended to accurately measure performance capability. Therefore, it is expected that the number of new connections offered for sale will decline in face of such stringent standards, but that real progress should accelerate because users can for the first time, begin immediate use of a new ISO qualified connection with confidence. Application for the instant invention is made with that realization in mind.
For purposes of the present invention, the following definitions will apply.
Flank angle=The acute angle in a plane coinciding with the pipe axis, measured between a thread flank and a plane positioned 90 degrees to the axis, the angle being plus if the flank faces away from the axis, the angle being minus if the flank faces toward the axis.
Thread Turn=A 360 degree portion of a screw thread.
Metal-to-metal seal=Continuos intimate contact of an annular non-threaded sealing surface around a portion of a pin, with a mating box surface, sufficient to effect a seal against fluid pressure within or outside of the connection.
Pin=A tapered male pipe end having a first thread turn of smallest diameter and a last thread turn of largest diameter, which may or may not include a metal-to-metal sealing surface adjacent one or both ends of the thread.
Box=A tapered female pipe end formed to mate with the pin, having a first thread turn of largest diameter and a last thread turn of smallest diameter, which may or may not include a metal-to-metal sealing surface adjacent one or both ends of the thread.
Pin Critical Area=The annular cross-sectional area of the pin, in a plane positioned normal to the axis, at the largest diameter of the engaged threads.
Box Critical Area=The annular cross-sectional area of the box, in a plane positioned normal to the axis, at the smallest diameter of the engaged threads.
Stab Flank=flank of the thread form facing toward that end of the pipe joint.
Load flank=flank of the thread form facing away from that end of the pipe joint.
Stab pitch=axial length between stab flanks, one thread turn apart.
Load pitch=axial length between load flanks, one thread turn apart.
Full-strength connection=a pipe connection that will seal and not rupture under any combination of mechanical loads and fluid pressure under which, the VME yield stress of the pipe is not exceeded.
Upset end=A pipe end that been plastically deformed axially so as to increase the wall thickness to a greater dimension than the original pipe wall thickness.
Swaged end=A pipe end that has been plastically reformed radially so as to: increase the outer diameter of the end to exceed the original pipe diameter; or to decrease the inner diameter of the end to be less than the original pipe inner diameter.
End-length=means a predetermined length of the pipe as measured from a pipe end.
The meaning of all words not specifically defined herein, may be found in Webster""s Dictionary.
It is well known in the art of threaded pipe connections, to upset the ends of a pipe joint for more than the length of threads to be later formed thereon, so as to increase the critical area of the pin and/or box. Allison U.S. Pat. No. 214,804 and Cummins U.S. Pat. No. 859,803 are early examples. Upsetting the ends of high strength steel pipe as used in Oil and Gas Wells requires very special machines that: heat the pipe end to above the lower critical temperature of the pipe material; grip the pipe adjacent it""s end very firmly; then impact the pipe end so as to forge the end to fill special form dies as required for each pipe size and wall thickness. High strength pipe joints must then be re-heat treated and restraightened, which further increases their cost and decreases their availability.
It is also well known to swage pipe ends having full thickness walls for more than the length of threads to be formed thereon, so as to allow box threads to be formed on a larger diameter than the original pipe diameter, to increase the resulting critical areas. However, when a box is swaged for such length, the wall thickness at the critical section is reduced to less than the pipe wall thickness, which reduces the box critical area and reduces the pressure rating of the box to below the pipe rating because it then has both a thinner wall and a larger diameter.
Watts U.S. Pat. No. 5,516,158 discloses the self-swaging of pipe ends for only a portion of the thread lengths upon assembly of the connection, which is a very useful improvement especially on large diameter pipe. However, if the diameter change upon such swaging becomes too large a percentage of the pipe diameter such that the thread form is substantially changed, then some of the advantage may be lost, such as may be the case for small diameter pipe.
It is well known that the wall thickness, the engaged thread length, the flank angles and the critical areas of a threaded pipe connection affect the mechanical and fluid pressure strengths of a threaded pipe connection. Also well known is that, the critical area of the box increases with a decrease in the smallest diameter of thread engagement, and that the critical area of the pin increases with an increase in the largest diameter of thread engagement.
The present invention discloses a box and pin that may be formed on plain-end pipe and a method to manufacture them, so as to increase their critical areas to as much as the pipe cross-sectional area selectively, and thereby provide a higher strength integral pipe connection than hitherto possible, without first upsetting or swaging the full-thickness pipe end wall before threading it. The method generally comprises: machining a predetermined end-length of the pipe to a desired first configuration having a reduced wall thickness; swaging the first configuration to a second desired configuration while swaging the other side of the wall to a desired third configuration; and then machining the pipe end-length to the desired final dimensions. The axial length of the reduced thickness end-length swaged may be equal, more, or less than the engaged thread length, as opposed to prior art that upsets or pre-swages the full thickness pipe wall over the full thread length or longer. An advantage of the present invention over my self-swaging connection cited above, particularly for small diameter pipe connections, is that a precise thread form is maintained after assembly of the present invention, because the thread form is machined after swaging. The preferred embodiment is described as follows.
To manufacture the box end: counterbore a predetermined end-length of a pipe joint to form a reduced thickness pipe wall having a desired first configuration within the pipe end; swage outwardly, the first configuration to form a desired second configuration and the outer pipe surface opposite the first configuration to form a desired third configuration larger in outer diameter than the original pipe outer diameter; and then finish machine the end-length to the desired box dimensions.
To manufacture the pin end: remove a desired end-length of a pipe joint outer wall so as to form a reduced thickness pipe wall of predetermined length having a desired outer first configuration; swage inwardly, the first configuration to effect a second desired outer configuration and a desired third configuration of the pipe bore opposite the second configuration; and then finish machine the end-length to the desired pin dimensions.
It should be understood that so swaging such thinner end-lengths takes only a small fraction of the time and force that conventional upsetting or swaging of full thickness pipe walls for the length of the engaged thread so therefore, the entire procedure may be done on the same machine tool that threads the pin and/or box ends, which greatly reduces cost and delivery time. Thus, pipe inventories may be reduced, which promotes substantial savings and availability. Depending on the pipe size and material characteristics, such swaging may be done cold or selectively, after heating to above the lower critical temperature as with an induction coil, when required by pipe dimensions, pipe material or customer specifications.
The configurations described above may comprise any combination of cylindrical, conical or curved portions. The scope of the present invention includes boxes and pins made with or without metal-to-metal seals, at either or both ends of the engaged threads. When metal-to-metal seals are required, the end-lengths are dimensioned longer sufficiently to provide for such seals, and the threads may or may not be dimensioned to seal, depending on the intended service requirements.
Any suitable thread form may be used with the present invention, however for maximum utility, the open wedge thread form of my co-pending application Ser. No. 09/573,480 or co-pending application PCT/US00/28829 is preferred, in that it provides a wide torque range even when machined on thin wall pipe, and will withstand very high mechanical and fluid pressure loads.
Thus, it is now clear that the present invention teaches how to manufacture a cost-effective high strength integral threaded pipe connection on plain-end pipe, that may be rated selectively, as high as a full strength connection.